Optical barriers, also called immaterial barriers, are used particularly to protect operators working near dangerous machines. These barriers are usually composed of a set of contiguous individual beams defining a detection plane. The beams are laid out systematically, since this is necessary to define a detection sensitivity. This sensitivity represents the ability of the barrier to detect intrusion of an obstacle into the barrier. For example, in tests it can be defined by simultaneously blanking off two beams. But it must be certain that the blanked off beams are not replaced by light falling on receivers originating from other beams.
Most barriers used at the present time use one or several light pulses for each beam, in order to setup an emitter receiver dialogue. These pulses are repeated identically for each beam in one or several barriers. The result is the following disadvantages that can result in serious weaknesses:
lack of certainty about the synchronization of beams between sending and receiving, PA1 risk of loss of sensitivity, PA1 possible interference when several barriers are installed close to each other, PA1 when a failure occurs, it is impossible to know which side is causing it (emitter or receiver). PA1 A--each barrier is identified by a number with hexadecimal digits, PA1 B--emitter and receiver elements associated with each other through their beam are grouped into modules with not more than 16 emitter and/or receiver elements, PA1 C--words are formed of binary pulses coded for their identification, and their work cycle is close to or equal to one. PA1 D--elements of modules of a barrier are coded to assign a word to each element (beam): PA1 the number of each module comprises any N=4 hexadecimal digits and the elementary number of positions in each module is equal to not more than 4 N=16; PA1 each word is 8 digits long and it is preceded and terminated by a word start bit and a word end bit.
The operation of a barrier may also be disturbed by parasite reflections of a beam, such that the beam, or a beam forming part of another barrier, is reflected onto its receiver.
These situations may become very dangerous if a barrier is thus completely or partially neutralized by parasite signals.
In general, the problem of reliability and consequently safety of a light barrier depends on the ability to know with certainty that the beam received by a receiver is actually the beam emitted by the emitter associated with it.
It would undoubtedly be possible to identify each beam by a unique code, but this type of solution would require calculation means incompatible with industrial and economic constraints.